IGCSE Physics 0625: Wavefronts, Wave Speed and Reflection in a Ripple Tank

In a ripple tank demonstration, a straight vibrating bar produces straight, parallel wavefronts that travel across the water and strike a flat metal barrier placed across one end of the tank.

(a) State what a wavefront represents in terms of the vibration of the water surface. [1]

(b) The vibrating bar is driven by a motor at a stated frequency of $5.0\text{ Hz}$ . Before reaching the barrier, the wavefronts are measured to be $3.0\text{ cm}$ apart. Calculate the speed of the waves in the ripple tank, giving your answer in m/s. [2]

(c) State what happens to the wavelength, frequency and speed of the waves as they reflect from the barrier, compared with their values before reflection, and describe one measurement the technician could make on the reflected wavefronts to confirm that the frequency has not changed. [3]

Part (a): What a wavefront represents

A wavefront is a line drawn through all the points on the water surface that are vibrating in phase with each other. That is, all at the same stage of their up-and-down oscillation at the same instant. A convenient way to picture this is a line joining a row of adjacent wave crests.

Part (b): Calculating the wave speed

The wavefront spacing is the wavelength of the waves. Converting to metres:

$\lambda = 3.0\text{ cm} = 0.030\text{ m}$

Using the wave equation with the stated driving frequency:

$v = f\lambda = 5.0\text{ Hz} \times 0.030\text{ m}$

$v = 0.15\text{ m/s}$

Part (c): Effect of reflection

When the waves reflect from the barrier, the water and the vibrating bar’s driving frequency have not changed, so:

The wavelength stays the same.
The frequency stays the same.
The speed stays the same.

Only the direction of travel of the waves reverses at the barrier.

To confirm the frequency is unchanged without touching the motor, the technician could re-measure the spacing between the reflected wavefronts. Since the water is unchanged, the wave speed is still $0.15\text{ m/s}$ ; finding the reflected spacing is still $3.0\text{ cm}$ shows, via $v = f\lambda$ , that the frequency must still be $5.0\text{ Hz}$ .

Final answers

(a) A wavefront is a line joining points vibrating in phase, e.g. a line of adjacent crests
(b) Wave speed $= \boxed{0.15\text{ m/s}}$
(c) Wavelength, frequency and speed are all unchanged; only the direction of travel reverses. Re-measuring the reflected wavefront spacing (still $3.0\text{ cm}$ ) confirms via $v = f\lambda$ that the frequency is unchanged